Amplitude balance circuit



Feb. 7, 1961 R. w. ALLEN ETAL 2,971,162

AMPLITUDE BALANCE CIRCUIT Original Filed Aug. 4, 1954 United StatesAMPLITUDE BALANCE CIRCUIT t Continuation of application Ser. No.447,788, Aug. 4, 1954. This application Mar. 18, 1958, Ser. N0. 722,352

4 Claims. (Cl. S30-116) This application is a continuation ofapplication 37,394, Serial No. 447,788, filed August 4, 1954, nowabandoned,

The present invention relates generally to differential signal gaincontrol systems and particularly to an improved differential amplitudebalance circuit which is useful in loran receiving equipment.

In conventional loran receivers master and slave loran pulses arereceived during separate reception intervals, and amplified in a timesharing amplifier such that the gain of the receiver may be separatelyadjusted for slave and master pulses. During the reception of thestronger of the received pulses the receiver gain is reduced toattenuate these pulses by applying 4a portion of a square wave theretoas a `gain control bias.

In loran gain control circuits it is essential that th positive half ofthe square wave gain control bias supplied' to the receiver be clampedto the receiver bias level in order that the weaker loran pulse may bereceived under optimum receiving conditions. A shunt diode used as aD.C. restorer is unsatisfactory for the purpose because appreciablechanges in D.,-C. clamping level occur due to the diode resistance. Aseries diode circuit has the disadvantage of long recovery time of thebias adjustment. Keyed diode clampers require extra tubes and morecomplicated circuitry.

'It is an object of the present invention to provide an improved signalgain or amplitude balance control system having none of the foregoingdisadvantages and yet which is relatively simple and inexpensive.

Another object is to provide an improved system for dierentiallycontrolling both the relative magnitudes and absolute magnitudes of aplurality of electrical signals.

A typical embodiment of a bias control circuit in accordance with theinvention comprises a push-pull amplifler including a pair of amplifyingdevices having their output circuits coupled to opposite ends of apotentiometer, the center tap of which is connected to a source ofreference potential. The amplifying devices are alternately driven tocutoff by lalternate half cycles of a square wave input signal. Aconstant voltage output bias signal is derived from the potentiometermovable contact when it coincides with the center tap. When the contactis moved in either direction from the center tap the amplitude and phaseof the amplifier square wave output bias voltage changes, the amplitudedepending upon the relative positions of the center tap andpotentiometer `arm andthe phase depending upon the direction ofdisplacement of the potentiometer arm from the center tap.`

In a preferred embodiment of the invention the potential of the centertap may be adjusted by a second potentiometer thus adjusting thequiescent bias level of the receiver during the reception of the weakerloran pulse.

In the preferred embodiment of the invention the pushpull amplifier isalso provided with means for compensating for differences in amplifyingdevices so that replacement of a burned-out amplifying device with vanew one will not affect circuit performance.

atent() i Patented Feb. 7, 1961 ice The foregoing and other objects,advantages and novel features of the invention will be more fullydescribed in connection with the `accompanying drawing the single figureof which is a schematic circuit diagram of a typical amplitude balancecircuit in accordance with the present invention.

Referring to the drawing, a square wave generator 11 supplies a pair ofoppositely phased square waves 12, 13. It is understood that when usedin a loran receiving system the square wave has a period L equal to theloran pulse period and a half period L/2. One of the square waves 12 iscoupled to the input (grid-cathode) circuit of a first amplifier tubethrough. a network including coupling capacitor 14, shunt resistor 16and series resistor 18.- The other square wave 13 is coupled to theinput circuit of a second amplier tube 21 through a similar networkincluding coupling capacitor 15, shunt resistor 17 and series resistor19.

The resultant push-pull amplifier output circuit includes apotentiometer 22 having a center tap A and a movable arm 22a. The centertap is connected to a point of reference potential, ground in thepresent circuit, through a second potentiometer 23. The circuit outputderived from the movable arm 22a is supplied to gain control elements ofthe receiver radio frequency and intermediate frequency stages in orderto control the gain of the receiver.

. In operation, the square waves supplied to the input circuit ofamplifier tubes 2t) and 21 are of suiicient amplitude alternately todrive the tubes to cutolf. Resistors 18 and 19 limit the grid currentsdeveloped across shunt resistors 16 and 17. In this manner, triodes 20and 21 act as switches, and minor variations on the input square wavessuch as loss of low frequency response due to the combination ofcondenser 14 and shunt resistor 16 and condenser 15 and shunt resistor17 are not reflected in the output.

If both triodes conduct equal currents during their on interval, thepotential at A is constant. The output signal, when arm 22a is set atany point on potentiometer section 22b, is negative with respect to tapA when triode 20 is conducting. When triode 2i) is not conducting thereis no current ow through potentiometer section 22b and therefore thepotential of arm 22a at any point along potentiometer section 22b is thesame as the potential at point A. Similarly, when triode 21 isconducting the output signal is negative with respect to A when arm 22ais setat any point on potentiometer section 22e. When triode 21 is notconducting there is no current flow through potentiometer section 22eand the potential at any point along the section is the same as that ofpoint A. In both cases, the amplitude of the output square wave signaldepends upon the relative position of arm 22a along section 22b 0r 22eand the output square wave taken from section 22h is always 180 out ofphase with the output square wave taken from section 22e.

The setting of potentiometer 23 determines the potential at point A.Thus, when potentiometer 23 is adjusted to minimum value, point A is atground potential and when it is adjusted to maximum value point A is ata maximum negative value, on the order of -20 to -25 volts in'anembodiment of the invention actually constructed. Potentiometer 23therefore adjusts the quiescent bias level of the receiver, that is, thebias level during the reception of the weaker of the loran. signals.Potentiometer 23 may be thought of as a bias or gain control andpotentiometer 22 as a balance control with perfect clamping to the biaslevel.

In a preferred embodiment of the invention in order to compensate fordifferences in tubes, shunt resistors 24a, 24b may be provided acrosspotentiometer 22. Adjustment of the arm of resistor 24b will balance thetriodeS 3 so that they conduct equally and provide the same potential atpoint A during both half cycles of square wave input. q

summarizing, the circuit described providesgthe desired timefsharin'gbalance control clamped to the gain setting used for the weaker receivedloran pulse without the use of clamping or D.C. restoring diodes.triodes are supplied with square waves 180 out of phase driving thetriodes from Zero to beyond cutoi. TheY output circuit of t-he triodesincludes va potentiometer opposite ends of which `are connectedtothepl-ates of the triodes. A xed center tap on the potentiometer isreturned to the value of bias desired for the reception of the weakerloran signal.

Although in the circuit described triode ampliers are employed, it is tobe understood' that the circuit is equally applicable to the use ofother amplifying devices such as tetrodes, pentodes or transistors.

What is claimed is:

1. In a hyperbolic navigation receiver of the type includin-g a stage towhich a gain control Voltage maybe applied for switching the receivergain between two discrete levels, .in'combination, a pair of switchmeans, said switch means passing current when closed and not passingcurrent when open, each having an inputterminal and an output terminal;a potentiometer, opposite ends of which are connected to said outputterminals, said potentiometer having a xed center tap and a movable tap;means providing a reference potential; meansfor varying the voltagedeveloped at said center tap including potentiometer means connectedbetween said center tap and said means providing said referencepotential; means connected to said input terminals for ialternatelyclosing and opening said switches and passing substantially the sameamounts of constant current through said switches. during the respectiveintervals they areclosed; and means forn connecting said movable tap tosaid stage of said hyperbolic navigation receiver for controlling thegain of said stage in accordance with the voltage developed at saidmovable tap.

2. In a hyperbolic navigation receiver of the type including a stage towhich a gain 4control voltage may be applied for switching the receivergain between two discrete levels, in combination, a push-pull ampliernc1ud- The grids of two ing a pair of amplifying devices, a pair ofinput circuits, one connected to each of said devices, and an outputcircuit including a potentiometer the opposite ends of which areconnected to said amplifying devices, and having a center tap and amovable tap; means for varying the voltage developed at said center tapincluding a second potentiometer connecting said center tap to a pointof reference potential; square wave generating means connected to saidpai; of input circuits for supplying 180 out-ofphase square wavevoltages thereto of sufcient amplitude `alternately to drive saiddevices to cut olf; and means for connecting said movable tap to saidstage of said hyperbolic navigation receiver for controlling the gain ofsaid receiver in accordance with the voltage developed at said movabletap.

3. The combination of a hyperbolic navigation receiver, the gain ofwhich it is desired to switch between two discrete levels: and a circuitfor producing a gain control voltage for said receiver, including apush-pull amplier having a pair of amplifying devices, a pair of inputcircuits, one connected to each of said devices, and

an output circuit including a potentiometer opposite ends of which areconnected to said amplifying devices, and having a fixed center tap landa movable tap; means connecting said center tap to a point of referencepotential; square wave generator means connected to said pair of inputcircuits for supplying like amplitude V outofphase square wave voltagesthereto of suicient amplitude alternately to drive'said devices to cutoff; and means connecting said movable tap to said receiver for applyinga gain control voltage thereto.

4. In thecombination as set forth in claim 3, further including meansfor adjusting the value of the voltage at said center tap, said meansincluding a potentiometer connected between said center tap and saidpoint of refer-V ence potential.

References Cited in the lle of this patent UNITED STATES PATENTS2,651,033 Frantz Sept. l, 1953 FOREIGN PATENTS 142,366 Australia July23, 1951

